land were only crudely adapted for terrestrial existence, but because they did
not encounter competitors, they survived.
Lobe-finned fish did, however, possess certain characteristics that
served them well in their new environment, including primitive lungs and
internal nostrils, both of which are essential for breathing out of the water.
Such characteristics, called preadaptations, did not develop because the others
were preparing to migrate to the land; they were already present by accident and
became selected traits only when they imparted an advantage to the fish on land.
The early land-dwelling amphibians were slim-bodied with fishlike tails,
but they had limbs capable of locomotion on land. These limbs probably developed
from the lateral fins, which contained fleshy lobes that in turn contained bony
elements.
The ancient amphibians never became completely adapted for existence on
land, however. They spent much of their lives in the water, and their modern
descendants, the salamanders, newts, frogs, and toads–still must return to
water to deposit their eggs. The elimination of a water-dwelling stage, which
was achieved by the reptiles, represented a major evolutionary advance.
The Reptilian Age
Perhaps the most important factor contributing to the becoming of
reptiles from the amphibians was the development of a shell- covered egg that
could be laid on land. This development enabled the reptiles to spread
throughout the Earth’s landmasses in one of the most spectacular adaptive
radiations in biological history.
Like the eggs of birds, which developed later, reptile eggs contain a
complex series of membranes that protect and nourish the embryo and help it
breathe. The space between the embryo and the amnion is filled with an amniotic
fluid that resembles seawater; a similar fluid is found in the fetuses of
mammals, including humans. This fact has been interpreted as an indication that
life originated in the sea and that the balance of salts in various body fluids
did not change very much in evolution. The membranes found in the human embryo
are essentially similar to those in reptile and bird eggs. The human yolk sac
remains small and functionless, and the exhibits have no development in the
human embryo. Nevertheless, the presence of a yolk sac and allantois in the
human embryo is one of the strongest pieces of evidence documenting the
evolutionary relationships among the widely differing kinds of vertebrates. This
suggests that mammals, including humans, are descended from animals that
reproduced by means of externally laid eggs that were rich in yolk.
The reptiles, and in particular the dinosaurs, were the dominant land
animals of the Earth for well over 100 million years. The Mesozoic Era, during
which the reptiles thrived, is often referred to as the Age of Reptiles.
In terms of evolutionary success, the larger the animal, the greater the
likelihood that the animal will maintain a constant Body Temperature independent
of the environmental temperature. Birds and mammals, for example, produce and
control their own body heat through internal metabolic activities (a state known
as endothermy, or warm-bloodedness), whereas today’s reptiles are thermally
unstable (cold-blooded), regulating their body temperatures by behavioral
activities (the phenomenon of ectothermy). Most scientists regard dinosaurs as
lumbering, oversized, cold-blooded lizards, rather than large, lively, animals
with fast metabolic rates; some biologists, however–notably Robert T. Bakker of
The Johns Hopkins University–assert that a huge dinosaur could not possibly
have warmed up every morning on a sunny rock and must have relied on internal
heat production.
The reptilian dynasty collapsed before the close of the Mesozoic Era.
Relatively few of the Mesozoic reptiles have survived to modern times; those
remaining include the Crocodile,Lizard,snake, and turtle. The cause of the
decline and death of the large array of reptiles is unknown, but their
disappearance is usually attributed to some radical change in environmental
conditions.
Like the giant reptiles, most lineages of organisms have eventually
become extinct, although some have not changed appreciably in millions of years.
The opossum, for example, has survived almost unchanged since the late
Cretaceous Period (more than 65 million years ago), and the Horseshoe Crab,
Limulus, is not very different from fossils 500 million years old. We have no
explanation for the unexpected stability of such organisms; perhaps they have
achieved an almost perfect adjustment to a unchanging environment. Such stable
forms, however, are not at all dominant in the world today. The human species,
one of the dominant modern life forms, has evolved rapidly in a very short time.
The Rise of Mammals
The decline of the reptiles provided evolutionary opportunities for
birds and mammals. Small and inconspicuous during the Mesozoic Era, mammals rose
to unquestionable dominance during the Cenozoic Era (beginning 65 million years
ago).
The mammals diversified into marine forms, such as the whale, dolphin,
seal, and walrus; fossorial (adapted to digging) forms living underground, such
as the mole; flying and gliding animals, such as the bat and flying squirrel;
and cursorial animals (adapted for running), such as the horse. These various
mammalian groups are well adapted to their different modes of life, especially
by their appendages, which developed from common ancestors to become specialized
for swimming, flight, and movement on land.
Although there is little superficial resemblance among the arm of a
person, the flipper of a whale, and the wing of a bat, a closer comparison of
their skeletal elements shows that, bone for bone, they are structurally similar.
Biologists regard such structural similarities, or homologies, as evidence of
evolutionary relationships. The homologous limb bones of all four-legged
vertebrates, for example, are assumed to be derived from the limb bones of a
common ancestor. Biologists are careful to distinguish such homologous features
from what they call analogous features, which perform similar functions but are
structurally different. For example, the wing of a bird and the wing of a
butterfly are analogous; both are used for flight, but they are entirely
different structurally. Analogous structures do not indicate evolutionary
relationships.
Closely related fossils preserved in continuous successions of rock
strata have allowed evolutionists to trace in detail the evolution of many
species as it has occurred over several million years. The ancestry of the horse
can be traced through thousands of fossil remains to a small terrier-sized
animal with four toes on the front feet and three toes on the hind feet. This
ancestor lived in the Eocene Epoch, about 54 million years ago. From fossils in
the higher layers of stratified rock, the horse is found to have gradually
acquired its modern form by eventually evolving to a one-toed horse almost like
modern horses and finally to the modern horse, which dates back about 1 million
years.
CONCLUSION TO EVOLUTION
Although we are not totally certain that evolution is how we got the way
we are now, it is a strong belief among many people today, and scientist are
finding more and more evidence to back up the evolutionary theory.
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